This invention relates to techniques for generating and detecting ultrasonic waves and, more particularly, to electromagnetic acoustic transducers.
Efficiency and economy have been increasingly emphasized in many areas of modern structural design, and this emphasis has stimulated the more widespread use of nondestructive testing techniques. Before nondestructive methods were developed, it was necessary to assume, in designing structural components, that flaws of a certain size were present in the construction materials. This design technique called for the selection of structural components which were sufficient in size and strength to function properly even when the assumed defects were present. Nondestructive testing methods, however, are capable of locating structural defects at an early stage in the life of a flaw, so that the appropriate corrective action, such as removing and replacing a defective component, can be accomplished before a defect causes a catastrophic failure. Consequently, where nondestructive testing can be implemented during their operational life, structural components may be manufactured and assembled more economically by reducing their dimensions and substituting lower strength, less expensive materials. Nondestructive inspection techniques can thus be utilized to maintain a desired level of reliability in a structure while concurrently reducing construction and materials costs.
One of the many types of nondestructive testing is ultrasonics, in which the interaction between acoustic wave energy and the internal structure of an object is analyzed to predict the physical integrity of the object. A key element in any ultrasonic nondestructive testing system is the transducer, which is used to convert electrical energy into acoustic wave energy in the test object and also to convert the acoustic energy back into electrical energy for detection purposes. Traditionally, the high conversion efficiency and modest cost of piezoelectric materials have led to their widespread use as ultrasonic transducers in many applications. Piezoelectric transducers are disadvantaged, however, by the need to be coupled to the ultrasonic medium through a liquid or solid bond.
Consequently, requirements for operation at high speeds, at elevated temperatures, in remote locations, with broadband and reproducible acoustic coupling, and without the subsequent cleanup of a liquid bond have spurred the development of noncontact ultrasonic techniques, such as electrostatic transducers, optical techniques, and electromagnetic transducers, which have supplanted piezoelectric transducers in many applications. One of the most promising noncontact transducers is the electromagnetic acoustic transducer (EMAT). An EMAT consists of a conductor which is positioned within a static magnetic field near the surface of a conducting material. When a radio frequency is applied to the conductor, eddy currents are induced in the material. If the magnetic field and the conductor are properly oriented, the Lorentz forces exerted on the eddy currents by the magnetic field will be transmitted to the lattice structure of the material and generate an ultrasonic wave. Reduced inspection time, an ability to operate in remote and inaccessible locations, and reduced transducer wear are some of the significant advantages offered by an EMAT-based nondestructive testing system.
EMATs have been fabricated with a variety of coil and magnet configurations to suit the requirements of particular applications. U.S. Pat. Nos. 3,850,028; 4,048,847; 4,080,836; 4,092,868; 4,104,922; 4,127,035; 4,184,374; 4,218,924; 4,232,557; and 4,248,092, for example, the teachings of which are incorporated herein by reference, illustrate some of the approaches which have been utilized. While EMATs have thus been employed to great advantage in many testing situations, some significant limitations of previous EMAT designs have been identified. The periodic permanent magnet EMAT, for example, which is best described in U.S. Pat. No. 4,127,035 and is illustrated herein in FIG. 1, is important because it can be used to generate certain types of ultrasonic waves, such as horizontally polarized shear (SH) waves, which are difficult or impossible to produce with other transducer designs. The fabrication of a periodic permanent magnet EMAT, however, requires extensive precision machine work to produce permanent magnets of the proper dimensions for the EMAT. In addition, the periodically varying magnetic field which is required is difficult to produce with electromagnets, although the use of electromagnets would be desirable in some applications because of the higher strength magnetic fields which could thereby be provided. Consequently, a need has developed for a new EMAT which will exhibit the advantages of the periodic permanent magnet EMAT while avoiding the limitations of that design.